Chiral phosphinoylalcohol complexes of monooxobis(peroxo)molybdenum(VI) and their use as asymmetric oxidants

Complexes of the type [MoO(O-2)(2)(L*)(ROH)], where L* = chiral beta-phosph inoylalcohol, have been synthesised and used as stoichiometric oxidants for a number of unfunctionalised alkenes. In all of the complexes the chiral a uxiliary is bound through the phosphinoyl oxygen as a monodentate ligand. T he coordination about the metal atom in these pseudo-pentagonal bipyramidal molecules is completed by a solvent molecule (ethanol/water) lying opposit e the axial Mo=O bond. Oxidation of small-chain non-functionalised alkenes occurs in variable yield to give epoxides with an enantiomeric excess of up to 40%. These compounds also behave as catalysts for the (BuOOH)-O-t oxida tions of alkenes, but with similar modest enantioselectivities. The modest enantioselectivities are explained on the basis of the mode of coordination of the chiral ligand, and it is argued that there may be inherent limits i n the use of these systems in asymmetric oxidations. (C) 1999 Elsevier Scie nce B.V. All rights reserved.